The present invention relates to a tonometer system for measuring intraocular pressure by accurately providing a predetermined amount of applanation to the cornea and detecting the amount of force required to achieve the predetermined amount of applanation. The system is also capable of measuring intraocular pressure by indenting the cornea using a predetermined force applied using an indenting element and detecting the distance the indenting element moves into the cornea when the predetermined force is applied, the distance being inversely proportional to intraocular pressure. The present invention also relates to a method of using the tonometer system to measure hydrodynamic characteristics of the eye, especially outflow facility.
The tonometer system of the present invention may also be used to measure hemodynamics of the eye, especially ocular blood flow and pressure in the eye's blood vessels. Additionally, the tonometer system of the present invention may be used to increase and measure the eye pressure and evaluate, at the same time, the ocular effects of the increased pressure.
Glaucoma is a leading cause of blindness worldwide and, although it is more common in adults over age 35, it can occur at any age. Glaucoma primarily arises when intraocular pressure increases to values which the eye cannot withstand.
The fluid responsible for pressure in the eye is the aqueous humor. It is a transparent fluid produced by the eye in the ciliary body and collected and drained by a series of channels (trabecular meshwork, Schlemm's canal and venous system). The basic disorder in most glaucoma patients is caused by an obstruction or interference that restricts the flow of aqueous humor out of the eye. Such an obstruction or interference prevents the aqueous humor from leaving the eye at a normal rate. This pathologic condition occurs long before there is a consequent rise in intraocular pressure. This increased resistance to outflow of aqueous humor is the major cause of increased intraocular pressure in glaucoma-stricken patients.
Increased pressure within the eye causes progressive damage to the optic nerve. As optic nerve damage occurs, characteristic defects in the visual field develop, which can lead to blindness if the disease remains undetected and untreated. Because of the insidious nature of glaucoma and the gradual and painless loss of vision associated therewith, glaucoma does not produce symptoms that would motivate an individual to seek help until relatively late in its course when irreversible damage has already occurred. As a result, millions of glaucoma victims are unaware that they have the disease and face eventual blindness. Glaucoma can be detected and evaluated by measuring the eye's fluid pressure using a tonometer and/or by measuring the eye fluid outflow facility. Currently, the most frequently used way of measuring facility of outflow is by doing indentation tonography. According to this technique, the capacity for flow is determined by placing a tonometer upon the eye. The weight of the instrument forces aqueous humor through the filtration system, and the rate at which the pressure in the eye declines with time is related to the ease with which the fluid leaves the eye.
Individuals at risk for glaucoma and individuals who will develop glaucoma generally have a decreased outflow facility. Thus, the measurement of the outflow facility provides information which can help to identify individuals who may develop glaucoma, and consequently will allow early evaluation and institution of therapy before any significant damage occurs.
The measurement of outflow facility is helpful in making therapeutic decisions and in evaluating changes that may occur with time, aging, surgery, or the use of medications to alter intraocular pressure. The determination of outflow facility is also an important research tool for the investigation of matters such as drug effects, the mechanism of action of various treatment modalities, assessment of the adequacy of antiglaucoma therapy, detection of wide diurnal swings in pressure and to study the pathophysiology of glaucoma.
There are several methods and devices available for measuring intraocular pressure, outflow facility, and/or various other glaucoma-related characteristics of the eye. The following patents disclose various examples of such conventional devices and methods:
PATENT NO.PATENTEE5,375,595Sinha et al.5,295,495Maddess5,251,627Morris5,217,015Kaye et al.5,183,044Nishio et al.5,179,953Kursar5,148,807Hsu5,109,852Kaye et al.5,165,409Coan5,076,274Matsumoto5,005,577Frenkel4,951,671Coan4,947,849Takahashi et al.4,944,303Katsuragi4,922,913Waters, Jr. et al.4,860,755Erath4,771,792Seale4,628,938Lee4,305,399Beale3,724,263Rose et al.3,585,849Grolman3,545,260Lichtenstein et al.
Still other examples of conventional devices and/or methods are disclosed in Morey, Contact Lens Tonometer, RCA Technical Notes, No. 602, December 1964; Russell & Bergmanson, Multiple Applications of the NCT: An Assessment of the Instrument's Effect on IOP, Ophthal. Physiol. Opt., Vol. 9, April 1989, pp. 212-214; Moses & Grodzki, The Pneumatonograph: A Laboratory Study, Arch. Ophthalmol., Vol. 97, March 1979, pp. 547-552; and C. C. Collins, Miniature Passive Pressure Transensor for Implanting in the Eye, IEEE Transactions on Bio-medical Engineering, April 1967, pp. 74-83.
In general, eye pressure is measured by depressing or flattening the surface of the eye, and then estimating the amount of force necessary to produce the given flattening or depression. Conventional tonometry techniques using the principle of applanation may provide accurate measurements of intraocular pressure, but are subject to many errors in the way they are currently being performed. In addition, the present devices either require professional assistance for their use or are too complicated, expensive or inaccurate for individuals to use at home. As a result, individuals must visit an eye care professional in order to check their eye pressure. The frequent self-checking of intraocular pressure is useful not only for monitoring therapy and self-checking for patients with glaucoma, but also for the early detection of rises in pressure in individuals without glaucoma and for whom the elevated pressure was not detected during their office visit.
Pathogens that cause severe eye infection and visual impairment such as herpes and adenovirus as well as the virus that causes AIDS can be found on the surface of the eye and in the tear film. These microorganisms can be transmitted from one patient to another through the tonometer tip or probe. Probe covers have been designed in order to prevent transmission of diseases but are not widely used because they are not practical and provide less accurate measurements. Tonometers which prevent the transmission of diseases, such as the “air-puff” type of tonometer also have been designed, but they are expensive and provide less accurate measurements. Any conventional direct contact tonometers can potentially transmit a variety of systemic and ocular diseases.
The two main techniques for the measurement of intraocular pressure require a force that flattens or a force that indents the eye, called “applanation” and “indentation” tonometry respectively.
Applanation tonometry is based on the Imbert-Fick principle. This principle states that for an ideal dry, thin walled sphere, the pressure inside the sphere equals the force necessary to flatten its surface divided by the area of flattening. P=F/A (where P=pressure, F=force, A=area). In applanation tonometry, the cornea is flattened, and by measuring the applanating force and knowing the area flattened, the intraocular pressure is determined.
By contrast, according to indentation tonometry (Schiotz), a known weight (or force) is applied against the cornea and the intraocular pressure is estimated by measuring the linear displacement which results during deformation or indentation of the cornea. The linear displacement caused by the force is indicative of intraocular pressure. In particular, for standard forces and standard dimensions of the indenting device, there are known tables which correlate the linear displacement and intraocular pressure.
Conventional measurement techniques using applanation and indentation are subject to many errors. The most frequently used technique in the clinical setting is contact applanation using Goldman tonometers. The main sources of errors associated with this method include the addition of extraneous pressure on the cornea by the examiner, squeezing of the eyelids or excessive widening of the lid fissure by the patient due to the discomfort caused by the tonometer probe resting upon the eye, and inadequate or excessive amount of dye (fluorescein). In addition, the conventional techniques depend upon operator skill and require that the operator subjectively determine alignment, angle and amount of depression. Thus, variability and inconsistency associated with less valid measurements are problems encountered using the conventional methods and devices.
Another conventional technique involves air-puff tonometers wherein a puff of compressed air of a known volume and pressure is applied against the surface of the eye, while sensors detect the time necessary to achieve a predetermined amount of deformation in the eye's surface caused by application of the air puff. Such a device is described, for example, in U.S. Pat. No. 3,545,260 to Lichtenstein et al. Although the non-contact (air-puff) tonometer does not use dye and does not present problems such as extraneous pressure on the eye by the examiner or the transmission of diseases, there are other problems associated therewith. Such devices, for example, are expensive, require a supply of compressed gas, are considered cumbersome to operate, are difficult to maintain in proper alignment and depend on the skill and technique of the operator. In addition, the individual tested generally complains of pain associated with the air discharged toward the eye, and due to that discomfort many individuals are hesitant to undergo further measurement with this type of device. The primary advantage of the non-contact tonometer is its ability to measure pressure without transmitting diseases, but they are not accepted in general as providing accurate measurements and are primarily useful for large-scale glaucoma screening programs.
Tonometers which use gases, such as the pneumotonometer, have several disadvantages and limitations. Such device are also subject to the operator errors as with Goldman's tonometry. In addition, this device uses freon gas, which is not considered environmentally safe. Another problem with this device is that the gas is flammable and as with any other aerosol-type can, the can may explode if it gets too hot. The gas may also leak and is susceptible to changes in cold weather, thereby producing less accurate measurements. Transmission of diseases is also a problem with this type of device if probe covers are not utilized.
In conventional indentation tonometry (Schiotz), the main source of errors are related to the application of a relatively heavy tonometer (total weight at least 16.5 g) to the eye and the differences in the distensibility of the coats of the eye. Experience has shown that a heavy weight causes discomfort and raises the intraocular pressure. Moreover the test depends upon a cumbersome technique in which the examiner needs to gently place the tonometer onto the cornea without pressing the tonometer against the globe. The accuracy of conventional indentation may also be reduced by inadequate cleaning of the instrument as will be described later. The danger of transmitting infectious diseases, as with any contact tonometer, is also present with conventional indentation.
A variety of methods using a contact lens have been devised, however, such systems suffer from a number of restrictions and virtually none of these devices is being widely utilized or is accepted in the clinical setting due to their limitations and inaccurate readings. Moreover, such devices typically include instrumented contact lenses and/or cumbersome and complex contact lenses.
Several instruments in the prior art employ a contact lens placed in contact with the sclera (the white part of the eye). Such systems suffer from many disadvantages and drawbacks. The possibility of infection and inflammation is increased due to the presence of a foreign body in direct contact with a vascularized part of the eye. As a consequence, an inflammatory reaction around the device may occur, possibly impacting the accuracy of any measurement. In addition, the level of discomfort is high due to a long period of contact with a highly sensitive area of the eye. Furthermore, the device could slide and therefore lose proper alignment, and again, preventing accurate measurements to be taken. Moreover, the sclera is a thick and almost non-distensible coat of the eye which may further impair the ability to acquire accurate readings. Most of these devices utilize expensive sensors and complicated electric circuitry imbedded in the lens which are expensive, difficult to manufacture and sometimes cumbersome.
Other methods for sensing pressure using a contact lens on the cornea have been described. Some of the methods in this prior art also employ expensive and complicated electronic circuitry and/or transducers imbedded in the contact lens. In addition, some devices use piezoelectric material in the lens and the metalization of components of the lens overlying the optical axis decreases the visual acuity of patients using that type of device. Moreover, accuracy is decreased since the piezoelectric material is affected by small changes in temperature and the velocity with which the force is applied. There are also contact lens tonometers which utilize fluid in a chamber to cause the deformation of the cornea; however, such devices lack means for alignment and are less accurate, since the flexible elastic material is unstable and may bulge forward. In addition, the fluid therein has a tendency to accumulate in the lower portion of the chamber, thus failing to produce a stable flat surface which is necessary for an accurate measurement.
Another embodiment uses a coil wound about the inner surface of the contact lens and a magnet subjected to an externally created magnetic field. A membrane with a conductive coating is compressed against a contact completing a short circuit. The magnetic field forces the magnet against the eye and the force necessary to separate the magnet from the contact is considered proportional to the pressure. This device suffers from many limitations and drawbacks. For example, there is a lack of accuracy since the magnet will indent the cornea and when the magnet is pushed against the eye, the sclera and the coats of the eye distort easily to accommodate the displaced intraocular contents. This occurs because this method does not account for the ocular rigidity, which is related to the fact that the sclera of one person's eye is more easily stretched than the sclera of another. An eye with a low ocular rigidity will be measured and read as having a lower intraocular pressure than the actual eye's pressure. Conversely, an eye with a high ocular rigidity distends less easily than the average eye, resulting in a reading which is higher than the actual intraocular pressure. In addition, this design utilizes current in the lens which, in turn, is in direct contact with the body. Such contact is undesirable. Unnecessary cost and complexity of the design with circuits imbedded in the lens and a lack of an alignment system are also major drawbacks with this design.
Another disclosed contact lens arrangement utilizes a resonant circuit formed from a single coil and a single capacitor and a magnet which is movable relative to the resonant circuit. A further design from the same disclosure involves a transducer comprised of a pressure sensitive transistor and complex circuits in the lens which constitute the operating circuit for the transistor. All three of the disclosed embodiments are considered impractical and even unsafe for placement on a person's eye. Moreover, these contact lens tonometers are unnecessarily expensive, complex, cumbersome to use and may potentially damage the eye. In addition none of these devices permits measurement of the applanated area, and thus are generally not very practical.
The prior art also fails to provide a sufficiently accurate technique or apparatus for measuring outflow facility. Conventional techniques and devices for measuring outflow facility are limited in practice and are more likely to produce erroneous results because both are subject to operator, patient and instrument errors.
With regard to operator errors, the conventional test for outflow facility requires a long period of time during which there can be no tilting of the tonometer. The operator therefore must position and keep the weight on the cornea without moving the weight and without pressing the globe.
With regard to patient errors, if during the test the patient blinks, squeezes, moves, holds his breath, or does not maintain fixation, the test results will not be accurate. Since conventional tonography takes about four minutes to complete and generally requires placement of a relatively heavy tonometer against the eye, the chances of patients becoming anxious and therefore reacting to the mechanical weight placed on their eyes is increased.
With regard to instrument errors, after each use, the tonometer plunger and foot plate should be rinsed with water followed by alcohol and then wiped dry with lint-free material. If any foreign material drys within the foot plate, it can detrimentally affect movement of the plunger and can produce an incorrect reading.
The conventional techniques therefore are very difficult to perform and demand trained and specialized personnel. The pneumotonograph, besides having the problems associated with the pneumotonometer itself, was considered “totally unsuited to tonography.” (Report by the Committee on Standardization of Tonometers of the American Academy of Ophthalmology; Archives Ophthalmol., 97:547-552, 1979). Another type of tonometer (Non Contact “Air Puff” Tonometer—U.S. Pat. No. 3,545,260) was also considered unsuitable for tonography. (Ophthalmic & Physiological Optics, 9(2):212-214, 1989). Presently there are no truly acceptable means for self-measurement of intraocular pressure and outflow facility.
In relation to an additional embodiment of the present invention, blood is responsible not only for the transport of oxygen, food, vitamins, water, enzymes, white and red blood cells, and genetic markers, but also provides an enormous amount of information in regards to the overall health status of an individual. The prior art related to analysis of blood relies primarily on invasive methods such as with the use of needles to draw blood for further analysis and processing. Very few and extremely limited methods for non-invasive evaluating blood components are available.
In the prior art for example, oxygenated hemoglobin has been measured non-invasively. The so called pulse oximeter is based on traditional near infrared absorption spectroscopy and indirectly measures arterial blood oxygen with sensors placed over the skin utilizing LEDs emitting at two wave lengths around 940 and 660 nanometers. As the blood oxygenation changes, the ratio of the light transmitted by the two frequencies changes indicating the amount of oxygenated hemoglobin in the arterial blood of the finger tip. The present systems are not accurate and provide only the amount of oxygenated hemoglobin in the finger tip.
The skin is a thick layer of tissue with a thick epithelium. The epithelium is the superficial layers of tissue and vary according to the organ or location in the body. The skin is thick because it is in direct contact with the environment and it is the barrier between the internal organs and the external environment. The skin is exposed and subject to all kind of noxious external agents on a daily basis. Stratified squamous keratinizing epithelium layers of the skin have a strong, virtually impermeable layer called the stratum corneum and keratin. The keratin that covers the skin is a thick layer of a hard and dead tissue which creates another strong barrier of protection against pathogenic organisms but also creates a barrier to the proper evaluation of bodily functions such as non-invasive blood analysis and cell analysis.
Another drawback in using the skin is due to the fact that the superficial layer of tissue covering the skin does not allow acquisition of important information, only present in living tissue. In addition, the other main drawback in using the skin is because the blood vessels are not easily accessible. The main vascular supply to the skin is located deep and distant from the superficial and still keratinized impermeable skin layer.
Prior art attempts to use the skin and other areas of the body to perform non-invasive blood analysis, diagnostics and evaluations of bodily functions such as oral, nasal and ear mucosa. These areas have been found to be unsuitable for such tasks. Moreover, placement of an object in oral or nasal mucosa can put the user at risk of aspiration and obstructing the airway which is a fatal event.
Another drawback in using the skin is the presence of various appendages and glands which prevent adequate measurements from being acquired such as hair, sweat glands, and sebaceous glands with continuous outflowing of sebum. Moreover, the layers of the skin vary in thickness in a random fashion. Furthermore, the layers of the skin are strongly attached to each other, making the surgical implantation of any device extremely difficult. Furthermore the skin is a highly innervated area which is highly sensitive to painful stimuli.
In order to surgically implant a device under the skin there is need for invasive application of anesthetic by injection around the area to be incised and the obvious risk of infection. Moreover, the structure of the skin creates electrical resistance and makes acquisition of electrical signals a much more difficult procedure.
Attempts to use electroosmosis as a flux enhancement by iontophoresis with increased passage of fluid through the skin with application of electrical energy, do not provide accurate or consistent signals and measurements due to the skin characteristics described above. Furthermore there is a significant delay in the signal acquisition when electroosmosis-based systems are used on the skin because of the anatomy and physiology of the skin which is thick and has low permeability.
Previously, a watch with sensing elements in apposition to the skin has been used in order to acquire a signal to measure glucose. Because of the unsuitable characteristics of the skin the watch has to actually shock the patient in order to move fluid. The fluid measured provides inconsistent, inaccurate and delayed results because of the unsuitable characteristics of the skin as described above. It is easy to see how unstable the watch is if one were to observe how much their own watch moves up and down and around one=s pulse during normal use. There is no natural stable nor consistent correct apposition of the sensor surface to the tissue, in this case the dead keratin layer of the thick skin.
Previously invasive means were used with tearing of the skin in the tip of the fingers to acquire whole blood, instead of plasma, for glucose measurement. Besides being invasive, whole blood from the fingers is used which has to be corrected for plasma levels. Plasma levels provide the most accurate evaluation of blood glucose.
The conventional way for blood analysis includes intense labor and many expenses using many steps including cumbersome, expensive and bulky laboratory equipment. A qualified medical professional is required to remove blood and this labor is certainly costly. The professionals expose themselves to the risk of acquiring infections and fatal diseases such as AIDS, hepatitis, and other viral and prion diseases. In order to prevent that possible contamination a variety of expensive measures and tools are taken, but still only providing partial protection to the medical professional and the patient. A variety of materials are used such as alcohol swabs, syringes, needles, sterile vials, gloves as well as time and effort. Moreover, effort, time and money must be spent with the disposal of biohazard materials such as the disposal of the sharps and related biohazard material used to remove blood. These practices negatively affect the environment as those biohazard materials are non-degradable and obviously made of non-recycled material.
In addition, these practices comprise a painful procedure with puncturing the skin and putting the patient and nurse at risk for infection, fatal diseases, contamination, and blood borne diseases. After all of this cumbersome, costly, time-consuming and hazardous procedure, the vials with blood have to be transported by a human attendant to the laboratory which is also costly. At the laboratory the blood is placed in other machines by a trained human operator with all of the risks and costs associated with the procedure of dealing with blood.
The conventional laboratory instruments then have to separate the blood using special and expensive machines and then materials are sent for further processing and analysis by a trained human operator. Subsequent to that the result is printed and sent to the patient and/or doctor, most frequently by regular mail. All of this process in laboratories is risky, complex, cumbersome, and expensive; and this is only for one test.
If a patient is admitted to a hospital, this very laborious and expensive process could happen several times a day. Only one simple blood test result can be over $100 dollars and this cost is easily explained by the labor and materials associated with the cost related to manipulation of blood and protection against infections as described above. If four tests are needed over 24 hours, as may occur with admitted patients, the cost then can increase to $400 dollars.
The world and in particular the United States face challenging health care costs with a grim picture of rapidly rising health care expenditures with a rapid increase in the number and frequency of testing. Today, the worldwide diabetic population alone is over 125 million and is expected to reach 250 million by the year 2008. The United States spent over $140 billion dollars on diabetes alone in 1998. More frequent control of blood glucose is known to prevent complications and would substantially reduce the costs of the disease.
According to the projections by the Health Care Financing Administration of the United States Department of Health and Human Services, health care spending as a share of U.S. gross domestic product (GDP) is estimated to increase from 13 percent to potentially and amazingly close to 20% of the United States GDP in the near future, reaching over $2 trillion dollars a year, which clearly demonstrates how unwise health care spending can affect the overall economy of a nation.
The World Health Organization reported in 1995, the percentage of total spending on health by various governments clearly indicating health care costs as a serious global problem and important factor concerning the overall utilization of public money. Public spending on health by the United States government was 47%, while United Kingdom was 84%, France was 81%, Japan was 78%, Canada was 71%, Italy was 70% and Mexico was 56%.
Infrared spectroscopy is a technique based on the absorption of infrared radiation by substances with the identification of said substances according to its unique molecular oscillatory pattern depicted as specific resonance absorption peaks in the infrared region of the electromagnetic spectrum. Each chemical substance absorbs infrared radiation in a unique manner and has its own unique absorption spectra depending on its atomic and molecular arrangement and vibrational and rotational oscillatory pattern. This unique absorption spectra allows each chemical substance to basically have its own infrared spectrum, also referred as fingerprint or signature which can be used to identify each of such substances.
Radiation containing various infrared wavelengths is emitted at the substance or constituent to be measured, referred to herein as “substance of interest”, in order to identify and quantify said substance according to its absorption spectra. The amount of absorption of radiation is dependent upon the concentration of said chemical substance being measured according to Beer-Lambert's Law.
When electromagnetic energy is emitted an enormous amount of interfering constituents, besides the substance of interest, are also irradiated such as skin, fat, wall of blood vessels, bone, cartilage, water, blood, hemoglobin, albumin, total protein, melanin, and various other interfering substances. Those interfering constituents and background noise such as changes in pressure and temperature of the sample irradiated drastically reduce the accuracy and precision of the measurements when using infrared spectroscopy. Those many constituents and variables including the substance of interest form then an absorption spectrum for each wavelength. The sum of the absorption for each wavelength of radiation by all of the constituents and variables generates the total absorption with said total absorption spectrum being measured at two or more wavelengths of emission.
In order then to achieve the concentration of the substance of interest, a procedure must be performed to subtract the statistical absorption spectra for each of the various intervening tissues and interfering constituents, with the exception of the substance of interest being measured. It is then assumed that all of the interfering constituents were accounted for and completely eliminated and that the remainder is the real spectra of the substance of interest. It has been very difficult to prove this assumption in vivo as no devices or methods in the prior art have yet shown to be clinically useful.
In the prior art the interfering constituents and variables introduce significant source of errors which are particularly critical since the background noise as found in the prior art tremendously exceeds the signal of the substance of interest which is found in minimal concentrations relative to the whole sample irradiated. Furthermore, in the prior art, the absorption of a solute such as glucose is very small compared to the other various interfering constituents which leads to many statistical errors preventing the accurate statistical measurement of glucose concentration. A variety of other techniques using infrared devices and methods have been described but all of them suffer from the same limitation due to the great amount of interference and noise.
Other techniques based on comparison with a known reference signal as with phase sensitive techniques have also the same limitations and drawbacks due to the great number of interfering constituents and generation of only a very weak signal. The interfering constituents are source of many artifacts, errors, and variability which leads to inadequate signal and severe reduction of the signal to noise ratio. Besides, calculation errors are common because of the many interfering substances and because the spectra of interfering constituents can overlap with the spectra of the substance of the interest being measured. If adequate signal to noise can be achieved, infrared spectroscopy should be able to provide a clinically useful device and determine the concentration of the substance of interest precisely and accurately.
Attempts in the prior art using infrared spectroscopy for noninvasive measurement of chemical substances have failed to accurately and precisely measure chemical substances such as for example glucose. The prior art have used transcutaneous optical means, primarily using the skin non-invasively, to determine the concentration of chemical substances. The prior art has also used invasive means with implant of sensors inside blood vessels or around the blood vessels. The prior art used polarized light directed at the aqueous humor of the eye, which is located inside the eye, in an attempt to measure glucose in said aqueous humor. However, precise measurements are very difficult to achieve particularly when there is substantial background noise and minimal concentration of the substance of interest as it occurs in the aqueous humor of the eye. Besides, polarized light techniques as used in the aqueous humor of the eye can only generate a very weak signal and there is low concentration of the solute in the aqueous sample. The combination of those factors and presence of interfering constituents and variables prevent accurate measurements to be achieved when using the aqueous humor of the eye.
The most frequent optical approaches in the prior art were based on measuring chemical substances using the skin. Other techniques include measuring substances in whole blood in the blood vessel (either non-invasively transcutaneously or invasively around or inside the blood vessel). Yet attempts were made to measure substances present in interstitial fluid with devices implanted under the skin. Attempts were also made by the prior art using the oral mucosa and tongue.
Mucosal surfaces such as the oral mucosa are made to stand long wear and tear as occurs during mastication. If the oral mucosa or tongue lining were thin with exposed vessels, one would easily bleed during chewing. Thus, those areas have rather thick lining and without plasma leakage. Furthermore these mucosal areas have no natural means for apposition of a sensor such as a natural pocket formation.
Since there is still a low signal with an enormous amount of interfering constituents, useful devices using the oral mucosal, tongue, and other mucosa such as genito-urinary and gastrointestinal have not been developed. The prior art also attempted to measure glucose using far infrared thermal emission from the body, but a clinically useful device has not been developed due to the presence of interfering elements and great thermal instability of the sample. Near infrared spectroscopy and far-infrared techniques have been tried by the prior art as means to non-invasively measure glucose, but accuracy and precision for clinical application has not been achieved.
Therefore remains a need to provide a method and apparatus capable of delivering a higher signal to noise by reducing or eliminating interfering constituents, noise, and other variables, which will ultimately provide the accuracy and precision needed for useful clinical application.